This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-155445, filed May 24, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a color cathode-ray tube having a shadow mask.
2. Description of the Related Art
In general, a color cathode-ray tube includes a glass vacuum envelope which comprises a substantially rectangular panel having an effective portion, a funnel connected to the panel, and a cylindrical neck connected to a small-diameter portion of the funnel. On the inner face of the effective portion of the panel is formed a phosphor screen which includes dot-shaped or striped three-color phosphor layers emitting blue, green, and red lights, and black color light shielding layers. Further, in the vacuum envelope, a shadow mask having many electron beam passage apertures is arranged to oppose the phosphor screen. A an inline type electron gun for emitting three electron beams arranged in a line is located in the neck. A deflecting yoke is mounted from the outer periphery of the neck to the outer peripheral surface of the funnel.
In the color cathode-ray tube having the above-described structure, the three electron beams emitted from the electron gun are deflected horizontally and vertically by means of horizontal and vertical deflecting magnetic fields that are generated by the deflection yoke. The phosphor screen is horizontally scanned at high frequency and vertically scanned at low frequency with the electron beams that are passed through the shadow mask, whereupon a color image is displayed. At this time, the electron beams emitted from the electron gun are projected onto the phosphor screen at a given angle of incidence, and are sorted by the shadow mask in accordance with the angle of incidence on the shadow mask. Therefore, the three electron beams correspond, in a one-to-one correspondence, to the respective colors of red, blue and green of the phosphor screen.
In the color cathode-ray tube described above, the shadow mask is formed in a gradually-curving dome shape, and has a substantially rectangular mask surface in which many electron beam passage apertures having a diameter of about 100 xcexcm are formed, and a skirt portion extending substantial perpendicularly from the peripheral edge of the mask surface toward the electron gun. Further, the shadow mask is fixed to a mask frame via the skirt portion, and the mask frame is removably supported via elastic supports at stud pins provided on the panel.
Usually, the shadow mask is formed in a desired shape by press molding a flat base mask, in which electron beam passage apertures are formed in advance. Further, the shadow mask is fixed to the mask frame by welding tongue portions, which are respectively formed at the corner portions and the substantially central portions of the long sides and short sides of the skirt portion, to the mask frame, thereby constituting a shadow mask assembly.
In the above-described color cathode-ray tube, in order to reduce doming of the shadow mask at the time of displaying high-intensity images, the shadow mask is formed of Invar material (an iron-nickel alloy) as a low thermal expansion material.
However, in a color cathode-ray tube to be used as a monitor of a computer or the like, reverse display such as window display or the like is general. In a 17 to 19 inch tube, electric current of 500 to 700 xcexcA is irradiated as an electron beam onto the shadow mask. Therefore, there is much heat generation of the shadow mask, and mask doming cannot be completely prevented.
When the color cathode-ray tube displays a high-intensity image, the directions of the electron beams, which have passed through the electron beam passage apertures of the shadow mask, are changed in the direction of the center of the screen due to the mask doming at the intermediate portion of the screen. Therefore, the beam spots of the electron beams formed on the phosphor screen cannot correctly land on the phosphor layer of the target color, and as a result, the color purity of the formed image deteriorates.
Further, the mask frame is mounted is formed of an inexpensive iron material, from the standpoint of costs. Therefore, the coefficients of thermal expansion of the mask frame and the shadow mask are about 10 times different from each other, and a difference arises in the amounts of thermal expansion of the shadow mask and the mask frame, at the time of high-luminance display.
When the shadow mask assembly is heated at 400 to 500xc2x0 C. in the manufacturing process, an extremely thin shadow mask is pulled by thermal expansion of the mask frame due to the difference in thermal expansions and the effective portion of the mask locally deforms particular at the vicinities of the welded points between the mask skirt portions and the mask frame. In consideration of such thermal expansion of the mask frame in the heating process, the diameter of the whole skirt portion of the shadow mask is formed larger than the inner diameter of the mask frame so that the skirt portion is pushed into the mask frame when the shadow mask is assembled to the frame, thereby preventing the shadow mask from being deformed in the heating process. In this case, however, as the color cathode ray tube displays high-luminance images, the effective portion of the mask locally deforms toward the electron gun due to the thermal expansion of the frame, in particular at the vicinities of the welded points of the long side portions and the short side portions of the shadow mask. In this case, at the peripheral portion of the screen, the directions of the electron beams, which have passed through the shadow mask, are changed further toward the outer peripheral direction of the screen than the target positions.
As described above, at the intermediate portion of the screen and at the peripheral portion of the screen, the landing positions of the electron beams which have passed through the shadow mask are changed in the opposite directions, and so-called landing reversal occurs. It is extremely difficult to completely suppress the deterioration in color purity caused by the landing reversal. Usually, attempts to balance the intermediate portion and the peripheral portion of the screen by correction by a holder or the like, have been carried out.
Further, in the case of the shadow mask to be used in the color cathode-ray tube with a flat screen, because the radius of curvature of the mask surface is large, the change in the landing position due to mask doming at the time of thermal expansion markedly increases. Therefore, with such a shadow mask, attempts to suppress mask doming and to suppress the deterioration in color purity by using an extremely low thermal expansion Invar material having a smaller coefficient of thermal expansion than that of Invar material, or the like, have been carried out.
However, the extremely low thermal expansion Invar is a material which is extremely hard to press-mold, and it is extremely difficult to mold a shadow mask having a large radius of curvature at a stable quality. Further, if an expensive material such as the extremely low thermal expansion Invar or the like is used as the shadow mask material which accounts for a large portion of the manufacturing costs of the color cathode-ray tube, the manufacturing costs greatly increase.
The present invention has been contrived in consideration of above-described points, and an object of the present invention is to provide a color cathode-ray tube which can suppress deterioration in color purity without causing an increase in costs.
In order to achieve the above-described object, a color cathode-ray tube according to an aspect of the present invention comprises: a panel having a phosphor screen on an inner surface of the panel; an electron gun located opposite the phosphor screen and configured to emit electron beams toward the phosphor screen; a shadow mask located opposite the phosphor screen and having a large number of electron beam passage apertures through which the electron beams pass; and a substantially rectangular mask frame supporting a peripheral portion of the shadow mask.
The shadow mask includes a substantially rectangular mask main surface having the electron beam passage apertures, and a skirt portion extending from a peripheral edge of the mask main surface in a direction substantially perpendicular to the mask main surface,
the mask main surface has a major axis extending perpendicular to a tube axis, a minor axis extending perpendicular to the tube axis and the major axis, and a diagonal axis extending through the tube axis,
the skirt portion has a pair of long side skirt portions positioned on both sides of the major axis, a pair of short side skirt portions positioned on both sides of the minor axis, and a plurality of notches extending from an extended edge of the skirt portion toward the peripheral edge of the mask main surface,
The notches are formed in the long side skirt portions and the short side skirt portions, and include a plurality of first notches defining first tongue portions respectively at substantially central portions of the respective long side skirt portions and substantially central portions of the respective short side skirt portions, and second notches provided as pairs so as to sandwich respective corner portions of the skirt portion and defining second tongue portions respectively, and the mask frame is disposed at the outside of the skirt portion and welded to the first and second tongue portions.
At the respective corner portions of the skirt portion, when a length from a peripheral edge of the mask surface to welding points of the second tongue portions is Lwd, and a length from the peripheral edge of the mask main surface to a distal end of at least one of the second notches is Lcd, Lcd and Lwd have the relationship of: Lcdxe2x89xa6Lwdxe2x89xa62Lcd.
Further, in accordance with a color cathode-ray tube according to another aspect of the present invention, the mask frame has a pair of long side walls facing the long side skirt portions and a pair of long side walls facing the short side skirt portions, and assuming that an angle between the major axis and a diagonal axis connecting a center of the mask frame and a diagonal point at which extensions of the long side walls and the short side walls intersect is xcex8ad, and an angle between the major axis and a diagonal welding axis connecting the welding point of the second tongue portion and the center of the mask frame is xcex8wd, xcex8ad and xcex8wd have the relationship of xcex8ad less than xcex8wd.
According to the color cathode-ray tube structured as described above, by setting the relationship between Lcd and Lwd to Lcdxe2x89xa6Lwdxe2x89xa62Lcd, due to thermal expansion of the mask frame, the tensile force applied to the diagonal welding points of the shadow mask is applied to the entire mask main surface, and mask doming is suppressed.
Further, in a laterally-long color cathode-ray tube whose screen size is 4:3 or 16:9, because mask doming is largest on the major axis X, by making the relationship be xcex8ad less than xcex8wd, the rigidity in the major axis direction at the corner portions of the skirt portion increases at the diagonal welding points. Therefore, the force of pulling the shadow mask applied in the major axis direction increases, and mask doming in the major axis direction can be effectively suppressed and landing reversal of electron beams at the intermediate and peripheral portions of the screen can be reduced.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.